<sstream> is not needed in exception free environment

[mborland]

Closes: #1412

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (0cfea2f) to head (bad351c).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1413   +/-   ##
========================================
  Coverage    95.39%   95.39%           
========================================
  Files          826      827    +1     
  Lines        68919    68921    +2     
========================================
+ Hits         65747    65749    +2     
  Misses        3172     3172           

Files with missing lines	Coverage Δ
include/boost/math/policies/error_handling.hpp	85.16% <ø> (ø)
test/git_issue_1412.cpp	100.00% <100.00%> (ø)

---

Continue to review full report in Codecov by Harness.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0cfea2f...bad351c. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mborland]

Spurious failure. Merging.